In drilling, testing, treating and cementing petroleum wells, it is sometimes desirable or even essential that a portion of the well bore be isolated from the rest. Generally, this may be accomplished by setting an hydraulically inflatable packer incorporated in a string of drill pipe in a cased or open well bore. In many instances, while an inflatable packer of the prior art can accomplish a sealing function against high pressure well fluids, it cannot provide anchoring force equivalent to slips. Under an applied load the packer begins to move or "creep" in the well bore. This phenomenon of "creep" occurs because the packer elastomers act as a highly viscous fluid under high stresses. As maintaining precise location in the well bore is important in many instances, for example when treating or testing a particular producing zone, a conventional elastomeric inflatable packer is unsuitable due to its extremely limited anchoring capabilities. Several prior art attempts have been made to develop an inflatable anchor packer, without a great deal of success.
For example, U.S. Pat. No. 2,970,651 discloses an hydraulically inflatable retrievable anchor utilizing spring steel slips having serrations on their exterior, the slips being held against the well bore wall by an inner elastomeric bladder. This approach suffers from a number of disadvantages, including the inability to seal between pipe string and well bore and the limitation of anchoring capability. The latter disadvantage is due to the limited holding capacity of the serrations (which would not be very effective in a cased well) or the undesirability of making the packer extremely long to provide sufficient frictional force with smooth bands and serrated exterior surfaces. Moreover, only the outer layer of bands contact the well bore, again causing a reduction in frictional anchoring capacity. Finally, such smooth bands do not easily retract when the packer is deflated and therefore hinder further movement in the well bore.
U.S. Pat. No. 3,035,639 discloses an inflatable retrievable packer possessing slips at its top and bottom which are molded or otherwise secured to an elastomeric packer bladder, being biased outwardly against the well bore or casing wall when the bladder is inflated. Unlike U.S. Pat. No. 2,970,651, a seal is provided between the pipe string and well bore or casing wall. However, the anchoring capability is severely restricted frictionally due to the limited length of the slips which are employed, as well as the fact that each set (upper and lower) of slips will hold against pressure in only one direction. Moreover, the ultimate limitation in anchoring capacity for this device is the tensile yield strength of the bladder elastomer, as the slips are bonded to it and not attached mechanically to the packer in any manner. While the drawing figures indicate that the slips may be backed on metal end rings, this is only true for the instance when the well bore is only slightly greater in diameter than the packer, a disadvantage for pipe string movement, particularly in deviated wells where the packer must traverse a curve in the bore.
Canadian Pat. No. 702,327 discloses several different anchoring devices in an inflatable retrievable packer, among them an abrasive particle coating on the packer bladder (FIGS. 1, 2, 8, 9, and 10), metallic buttons embedded in the bladder (FIGS. 4 and 5) and serrated metallic jaws embedded in the bladder (FIGS. 6 and 7). All of these combinations possess a marked limitation in frictional engagement ability, as well as an ultimate anchoring strength limitation of the tensile yield strength of the bladder elastomer.
In addition to those instances when it is necessary or desirable to maintain location of a packer in a well bore, there are also many occasions during the inspection and repair of pipelines when it is necessary to seal off the bore of the pipeline at a given location. For example, a pipeline may require sealing at a certain point in order to patch a leak or allow replacement of a section. In other instances, the pipeline bore may be sealed and the pressure raised behind the sealing device in order to ascertain if a leak exists in a particular pipeline section, the presence of such leak being indicated by a failure of the pipeline to hold pressure. Several prior art approaches have been taken in the design of such sealing devices.
U.S. Pat. No. 3,593,749 discloses a pipeline pig stopper which relies on mechanical grippers or slips to fix the pig in the pipeline bore, after which an inflatable element is expanded to effect the actual seal. Major disadvantages of such a device include the limitation of gripping power due to the relatively small size and rigid configuration of the grippers, and damage incurred by the interior of the pipe in which the grippers are set. These grippers may not conform exactly to the pipeline bore wall, which may be lined with relatively soft plastics. Such plastics can be easily damaged by the gripper serrations, particularly if the grippers are not perfectly aligned with the bore wall.
U.S. Pat. No. 4,077,435 discloses a pipeline plugging apparatus which relies on stopping mechanisms inserted in the wall of the pipeline to arrest plug movement, subsequently sealing the pipeline bore with an expandable elastomer seal. Whie effective, the use of such stopping mechanisms necessarily limits the use of such plugs to instances where the pipeline is easily accessible and where a leak has already been located.
U.S. Pat. No. 4,079,755 discloses a rubber-covered inflatable pipeline plug reinforced with tire cord to withstand high inflation pressures. The utilization of an elastomer necessarily limits the anchoring force obtainable by the device, as the elastomer will tend to "creep" at high pipeline pressures, acting like a highly viscous fluid. Such "creep" is extremely undesirable, particularly when an exact pipeline location is desired for repair or leak location purposes.
An inflatable pipeline plug employing overlapping smooth metal bands as an anchor element is also known, with a rubber sealing element employed over a portion of the metal bands. However, such a device is not reusable due to the "set" that the bands take when the plug is expanded.